1. Field of the invention
This invention relates to a coating apparatus for coating a coating liquid on a running support and, more particularly, to a coating apparatus for coating a magnetic dispersoid on a flexible support.
2. Discussion of the Related Art
Recently, to meet a demand for increased thin film coating speed, extrusion type coating apparatuses have been adopted in various fields (such as those according to the inventions disclosed in Japanese Patent Laid-Open Publications No. Sho 58-104666, No. Sho 60-238179, No. Sho 62-117666 and No. Hei 2-265672). These extrusion type coating apparatuses have their head (hereinafter referred to as die head) provided with an upstream and a downstream lip. A slot is defined between these lips such that a coating liquid (for instance a magnetic dispersoid) can be extruded through it to be coated on a support running past the upstream and downstream lips.
FIGS. 8A and 9A show prior art extrusion type coating apparatuses. As shown, the apparatus 1 or 11 has an upstream lip 2 or 12 and a downstream lip 3 or 13. The downstream lip 3 or 13 has an end surface 7 or 17 (coating operation surface) having a straight or single arcuate sectional (constant radius of curvature) profile. One of the reasons for this shape is simplicity of processing. More specifically, the downstream lip 3 or 13 is made of a super-hard alloy or like very hard material, and high machining accuracy (in the order of sub-microns) is required for it. For this reason, the coating operation surface of the downstream lip 3 or 13 is limited to a planar or single arcuate surface. Reference numerals 4 and 14 in FIGS. 8A and 9A designate slots.
The coating operation surface 7 or 17 of the downstream lip 3 or 13, however, is very closely related to coating operation conditions such as the coating speed, coating film thickness, kind of coating liquid, etc. Therefore, the shape of the coating operation surface has to be determined by taking these coating operation conditions into consideration.
For example, in the coating apparatus 1 in which the downstream lip 3 has a flat coating operation surface 7, as shown in FIGS. 8A and 8B, the pressure of the coating liquid 6 which is extruded into the space between the coating operation surface 7 of the downstream lip 3 and the support 5, and which acts on the support 5, is concentratedly at a support position facing to the upstream end of the coating operation surface 7 (point B), i.e., a position on the support which is intersected by a plane forming an extension of the downstream wall of the slot 4 and passing through the point B and at a support position facing to a position immediately upstream of the downstream end (point C). The pressure of the coating liquid 6 must be set to be above a predetermined value at the support position facing to the point B in order to prevent the air entraining into the coating film. However, for the other positions it is desirable for the liquid pressure to be low for reducing the load acting on the support 5. For this reason, in this coating apparatus 1, the ranges of the coating operation conditions which permit satisfactory coating are very narrow.
In the coating apparatus 11 in which the downstream lip 13 has a single arcuate coating operation surface 17, as shown in FIGS. 9A and 9B, the pressure of the coating liquid 6 extruded into the space between the coating operation surface 7 of the downstream lip 13 and the support 5 and acting on the support 5 is not concentrated at the support position facing to a position immediately upstream of the point C, but unnecessary pressure is produced between the points B and C. For this reason, when the coating operation conditions are changed slightly, the coating liquid pressure is changed as shown by the dashed curve in FIG. 9B to increase the load acting on the support 5. Therefore, in the case of the coating apparatus 11, the ranges of the coating operation conditions that permit satisfactory coating are again narrow.